1. Field of the Invention
The present invention relates to an apparatus and method for calibrating an array antenna.
2. Description of the Related Art
To form an accurate receiver beam in a digital beam forming device, it is necessary in beam forming to make uniform the amplitude characteristics and the phase characteristics of the outputs of receivers provided to antenna elements respectively.
An array antenna calibration apparatus is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2000-151255 (Method and Apparatus for Calibrating Array Antenna) and Jpn. Pat. Appln. KOKAI Publication No. Hei 10-336149 (Array Antenna radio CDMA Communication Apparatus). The configuration of one example of conventional array antenna calibration apparatuses is shown in FIG. 5.
In this array antenna calibration apparatus, between antenna elements 801-2 through 801-5 and receivers 802-1 through 802-4 are provided couplers 821-1 through 821-4 respectively, so that a calibration signal generated by a calibration signal generator 810 is divided by a divider 809. Thus divided calibration signals are input from the couplers 821-1 through 821-4 to the receivers 802-1 through 802-4 respectively. The calibration signals thus received by the receivers 802-1 through 802-4 undergo propagation factor estimation at propagation factor estimators 808-1 through 808-4 of a calibration signal processor 806 respectively, which output propagation factors to a calibration factor calculator 805. The calibration factor calculator 805 then calculates a calibration factor based on the propagation factors so that the amplitudes and phases of the signals from the receivers 802-1 through 802-4 may be equal respectively. Thus obtained calibration factor is input to beam former 803 of each user, and the beamformer 803 correct their respective output signals from the receivers 802-1 through 802-4 according to the calibration factor.
In such a conventional calibration apparatus, the calibration signals do not pass through the antenna elements 801-2 through 801-5 nor interconnections between them and the couplers 821-1 through 821-4, so that it cannot correct fluctuations in characteristics caused by these components, which is a problem. Furthermore, in the conventional calibration apparatus, when the calibration signals are input to the receivers 802-1 through 802-4, they must be equal in both amplitude and phase. This necessity gives rise to a problem that the divider 809 and the couplers 821-1 through 821-4 must have performance of high accuracy and high stability.
To solve these problems, there has been disclosed such a conventional method as shown in FIG. 6. This conventional method is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2000-295152 (Array Antenna Radio Communication Apparatus). By this calibration method, a calibration signal generator 810 is installed at a position where there is no obstacle to an array antenna at a base station, in order to transmit a calibration signal therefrom to the base station array antenna. By this calibration method, the calibration signal is received by the antenna elements 801-2 through 802-5 and the receivers 802-1 through 802-4 for calibration. The calibration signal can pass through from the antenna elements 801-1 through 801-5 to the receivers 802-1 through 802-4 all the way for calibration. The method, however, has a problem that the calibration signal generator must be installed within an unobstructed range of the base station. Furthermore, it has another problem that it is necessary to know an accurate positional relationship between the base station and the signal generator.
In view of the above, it is an object of the present invention to provide an array antenna calibration apparatus and method which can take into account the characteristics of a propagation factor ranging from an antenna element to a receiver and also which eliminates the necessity of knowing a positional relationship between a base station and a signal generator.
The present invention provides a novel calibration apparatus and method which calibrates the reception characteristics of a linear array antenna used at the base station. A configuration of the apparatus of the present invention is described with reference to FIG. 1.
The array antenna calibration apparatus of the present invention comprises a plurality of antenna elements 1-2 through 1-5 which makes up an array antenna, receivers 2-1 through 2-4 connected to said antenna elements respectively, propagation factor estimators 4-1 through 4-4 which estimate propagation factors of user signals output from said receivers 2-1 through 2-4 respectively, antenna elements 1-1 and 1-6 which send a calibration signal to said array antenna, a calibration signal supplier 30-1 which transmits an equi-amplitude/equi-phase calibration signal from said antenna elements 1-1 and 1-6, a calibration factor supplier 40 which has means for obtaining a relative phase fluctuation and a relative amplitude fluctuation between said array antenna and said antenna elements, and a beam former 3 which calibrates said user signal received by each of said antenna elements of said array antenna based on said relative phase fluctuation and said relative amplitude fluctuation.
Furthermore, the calibration signal supplier 30-1 has a calibration signal generator 10 and a divider 9 for transmitting an equi-amplitude/equi-phase calibration signal, for supplying the calibration signal to the antennas 1-1 and 1-6 added to the two ends of the array antenna respectively so that the phase characteristics and the amplitude characteristics of outputs of the receivers 2-1 through 2-4 connected to the antenna elements 1-2 through 1-5 respectively may be made uniform.
Furthermore, a calibration factor supplier 40-1 is comprised of a calibration signal processor 6 which processes the calibration signal received by the antenna elements 1-2 and 1-5 at the two ends of the array antenna to which the receivers 2-1 and 2-4 are connected respectively and a calibration factor calculator 5 which calculates a calibration factor using the information of a phase difference of the calibration signal sent from the calibration signal processor 6 and a transmission path estimate value sent from each of the transmission path estimators 4-1 through 4-4 of each user. In this configuration, the calibration factor supplier 40-1 obtains a relative phase fluctuation and a relative amplitude fluctuation between the antenna elements of the array antenna based on the calibration signal received by the antenna element and the user signal received by each of the antenna elements of the array antenna, thus sending the calibration factor to the beam former 3.
The following will describe a calibration method of the present invention. The calibration signals transmitted from the antenna elements 1-1 and 1-6 are received by the receivers 2-1 and 2-4 through the antenna elements 1-2 and 1-5, respectively, owing to electromagnetic coupling between the antenna elements. The calibration signals received by the receivers 2-1 and 2-4 are sent to propagation factor estimators 8-1 and 8-2 of the calibration signal processor 6 respectively for estimation of their respective propagation factors.
The resulting propagation factors are used by a phase difference calculator 7 of the calibration signal processor 6 to calculate a phase difference between the outputs of the receivers 2-1 and 2-4 and then send it to the calibration factor calculator 5.
Furthermore, the user signals are received through the antenna elements 1-2 through 1-5 and the receivers 2-1 through 2-4 in this order and sent to the propagation factor estimators 4-1 through 4-4, where propagation factors of these user signals received at the antenna elements are estimated and output as a propagation factor. Thus given propagation estimate value is sent to the beam former 3 to be used to form a user-specific beam and also sent to the calibration factor calculator 5.
The calibration factor calculator 5 then uses the phase difference between the calibration signals and the user-specific propagation factor at each of the antenna elements, thus calculating a calibration factor for the output of each of the receivers 2-1 through 2-4. In calculation of the calibration factor, it is not necessary to use the propagation factors of all the users but they may be selected as many as an arbitrary number. Furthermore, the calibration factor obtained by the calibration factor calculator 5 is posted sent to the beam former 3 of each of the users, to be used there in order to correct the reception signal output from each of the receivers 2-1 through 2-4 for beam formation.
As described above, the present invention features that a user signal received and a calibration signal supplied through inter-antenna element coupling are used to make uniform the amplitude and phase characteristics of the receivers for calibration of the antenna.
The present invention provides an array antenna calibration apparatus comprising:
a plurality of first antenna elements in said antenna elements for calibration;
a calibration signal supplier for supplying a calibration signal to a second antenna element near at least two said first antenna elements of said array antenna, or a coupler connected to said first antenna element;
a calibration factor supplier for obtaining a relative phase fluctuation and a relative amplitude fluctuation between said antenna elements of said array antenna based on the calibration signal received by said at least two first antenna elements and user signals received respectively by said antenna elements of said array antenna; and
a beam former for calibrating said user signals received respectively by said antenna elements of said array antenna using said relative phase fluctuation and said relative amplitude fluctuation.
The calibration factor supplier may, but not necessarily, include:
apparatus for obtaining a propagation factor relating to said calibration signal for each of said first antenna elements based on said calibration signal received by each of said first antenna elements;
apparatus for obtaining a first phase difference of said propagation factor relating to said calibration signal between said first antenna elements based on said propagation factor;
apparatus for obtaining an average of said phase differences of said propagation factors relating to said calibration signals between said first antenna elements based on said phase differences of said propagation factors;
apparatus for obtaining an average of the propagation factors relating to said user signals for each of said antenna elements of said array antenna based on said user signals;
apparatus for obtaining a phase difference between said antenna elements caused by a difference in length of arrival paths based on said average of said phase differences and said average of said propagation factors;
apparatus for obtaining a first time-average of the relative phase fluctuations of each of said antenna elements with respect to the one of said first antenna elements as a reference, based on said average of said propagation factors relating to said user signals for each of said antenna elements of said array antenna, said average of said propagation factors relating to said user signals for each of said first antenna elements, and said phase difference between said antenna elements caused by the difference in length of said arrival paths; and
apparatus for obtaining a second time-average of the relative phase fluctuations of said first antenna which are not used as a reference, based on the phase difference of said propagation factors relating to said calibration signal between said first antenna elements.
The calibration factor supplier may, but not necessarily, include:
apparatus for obtaining the propagation factor relating to said calibration signal for each of said first antenna elements based on said calibration signal received by each of said first antenna elements;
apparatus for obtaining the phase difference of said propagation factor relating to said calibration signal between said first antenna elements based on said propagation factor relating to said calibration signal;
apparatus for obtaining the average of said phase differences of said propagation factors relating to said calibration signals between said first antenna elements based on said phase differences of said propagation factors;
apparatus for obtaining the average of said propagation factors relating to said user signals for each of said antenna elements of said array antenna;
apparatus for obtaining the phase difference between said antenna elements caused by the difference in length of the arrival paths based on said average of said phase differences of said propagation factors and said average of said propagation factors; and
apparatus for obtaining a time-average of the relative amplitude fluctuations for each of said antenna elements of said array antenna with respect to one of said antenna elements of said array antenna based on said average of the propagation factors.